FIG. 8 is a circuit diagram showing one exemplary conventional CMOS (Complementary Metal Oxide Semiconductor) photoelectric conversion circuit (so-called CMOS sensor).
In the CMOS sensor shown in FIG. 8, a photodiode 71 has an anode connected to a ground end. Photodiode 71 has a cathode connected to one end of a switch 74. The other end of switch 74 is connected to one end of a capacitor 72, the gate of an N-channel field-effect transistor 73, and one end of a switch 75. The other end of capacitor 72 is connected to a ground end. The other end of switch 75 is connected to an end to which power source voltage VDD is applied. Transistor 73 has a drain connected to an end to which power source voltage VDD is applied. Transistor 73 has a source connected to one end of a switch 76. The other end of switch 76 is connected to a received-light signal output line 77.
FIG. 9 is a vertical cross sectional view showing a device structure of the exemplary conventional CMOS photoelectric conversion circuit.
In the CMOS sensor shown in FIG. 8 and FIG. 9, upon its initialization, switch 74 is brought into the off state, and switches 75, 76 are both brought into the on state. By controlling the switches in this way, capacitor 72 is charged by charging current iy flowing via switch 75, thus increasing terminal voltage Vc thereof to a predetermined initial voltage level (i.e., fully charged level of capacitor 72). As a result, transistor 73 is reset to the initial state (full-on state) and output current iz flowing in received-light signal output line 77 has a maximal value.
Upon exposure of photodiode 71 after the initialization of the CMOS sensor, switch 74 is brought into the on state, and switches 75, 76 are both brought into the off state. By controlling the switches in this way, capacitor 72 is discharged by photo current ix obtained according to an amount of light received by photodiode 71, thus decreasing terminal voltage Vc thereof from the initial voltage level. As a result, depending on the amount of light received by photodiode 71, transistor 73 is brought into a more closed state (state in which on-resistance is increased and conductivity is decreased) than the initial state.
Upon reading of a received-light signal after the exposure of photodiode 71, switches 74, 75 are both brought into the off state and switch 76 is brought into the on state. By controlling the switches in this way, output current iz is drawn from received-light signal output line 77 in accordance with the conductivity of transistor 73 (i.e., the amount of light received by photodiode 71). Hence, the amount of light received by photodiode 71 can be detected based on an amount of reduction of output current iz.
As one example of conventional techniques associated with the above, Japanese Patent Laying-Open No. 2002-171142 (Patent Document 1), provided by the applicant of the present application, discloses the following technique. That is, it includes: a photoelectric conversion unit that generates current according to intensity of incident light; a logarithmic conversion unit that generates voltage through logarithmic conversion of the current generated by the photoelectric conversion unit; an offset unit that shifts by a predetermined value the voltage generated by the logarithmic conversion unit; a field-effect transistor having a gate and a source between which the voltage obtained by the logarithmic conversion unit and the voltage obtained by the offset unit are applied; and a current/voltage conversion unit that allows drain current of the transistor to flow in a resistor to convert it into voltage.
Japanese Patent Laying-Open No. 2004-159155 (Patent Document 2), also provided by the applicant of the present application, discloses the following technique: an area image sensor having a multiplicity of pixels forming pixel rows/pixel columns. As components of each of the pixels, the area image sensor includes: a photoelectric conversion element; a reset transistor that enables/disables a pixel signal of the photoelectric conversion element; a sense amplifier that amplifies the enabled pixel signal and outputs it; and a switching transistor that switches on/off the operation of the sense amplifier. In each of the pixels, the area image sensor includes: at least one capacitor provided between the photoelectric conversion element and the reset transistor to temporarily store the pixel signal; and at least one transfer transistor provided between the photoelectric conversion element and the capacitor to transfer the pixel signal to the capacitor and switch on/off for the transferring.
Patent Document 1: Japanese Patent Laying-Open No. 2002-171142
Patent Document 2: Japanese Patent Laying-Open No. 2004-159155